JST, CREST, and Department of Chemistry, Graduated School of Science Osaka City University, Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan.
Chemistry. 2013 Mar 4;19(10):3330-9. doi: 10.1002/chem.201204323. Epub 2013 Feb 12.
A series of quadruple-stranded Na(+) and Ca(2+) complexes with octadentate cyclen ligands was synthesized to produce complexes that contained four different side-arm combinations (one triazole-coumarin group and three pyridine groups (1), four pyridine groups (2), one triazole-coumarin group and three quinoline groups (3), and four quinoline groups (4)). X-ray crystallographic analysis revealed that no significant changes occurred in the stereostructure of these complexes upon replacing one pyridine group with a triazole-coumarin moiety, or by replacing Na(+) ions with Ca(2+) ions, although the coordination number of the complexes in the solid state decreased when pyridine groups were replaced by quinoline groups. In solution, all of the side arms were arranged in a propeller-like pattern to yield an enantiomer pair of Δ and Λ forms in each metal complex. The addition of a tert-butoxycarbonyl (Boc)-protected amino acid anion, that is, a coordinative chiral carboxylate anion, to the cyclen-Ca(2+) complex induced circular dichroism (CD) signals in the aromatic region by forming a 1:1 mixture of diastereomeric ternary complexes with opposite complex chirality, whilst the corresponding Na(+) complexes rarely showed any response. In complexes 1-Ca(2+) and 3-Ca(2+), this chirality-transfer process was efficiently followed by considering the induction of the CD signals at two different wavelengths, that is, the coumarin-chromophore region and the aza-aromatic region. The sign and intensity of the CD signal were significantly dependent on both the nature of the aza-aromatic moiety and the enantiomeric purity of the external anion. These Ca(2+) complexes worked as effective probes for the determination of the enantiomeric excess of the chiral anion. The cyclen-Ca(2+) complexes also interacted with the non-coordinative Δ-TRISPHAT anion through an ion-pairing mechanism to achieve chirality transfer from the anion to the metal complex; both complexes 1-Ca(2+) and 3-Ca(2+) clearly showed induced CD signals in the coumarin-chromophore region, owing to ion-paring interactions with the Δ-TRISPHAT anion. Thus, the proper combination of an octadentate cyclen ligand and a metal center demonstrated effective chirality transfer.
一系列具有八齿环戊二烯配体的 Na(+)和 Ca(2+)四重链配合物被合成出来，以产生包含四种不同侧臂组合的配合物（一个三唑-香豆素基团和三个吡啶基团 (1)，四个吡啶基团 (2)，一个三唑-香豆素基团和三个喹啉基团 (3)，和四个喹啉基团 (4)）。X 射线晶体学分析表明，当用三唑-香豆素部分替换一个吡啶基团，或者用 Ca(2+)离子替换 Na(+)离子时，这些配合物的立体结构没有发生显著变化，尽管当吡啶基团被喹啉基团取代时，配合物在固态中的配位数减少。在溶液中，所有的侧臂都排列成桨状，在每个金属配合物中产生对映体 Δ 和 Λ 形式的对映体。向环戊二烯-Ca(2+)配合物中加入叔丁氧羰基（Boc）保护的氨基酸阴离子，即配位手性羧酸根阴离子，通过形成具有相反配合物手性的非对映异构体三元配合物的 1:1 混合物，在芳香区域诱导圆二色性 (CD) 信号，而相应的 Na(+)配合物很少有任何响应。在 1-Ca(2+)和 3-Ca(2+)配合物中，通过考虑在两个不同波长处诱导 CD 信号，可以有效地进行这种手性转移过程，即香豆素发色团区域和氮杂芳族区域。CD 信号的符号和强度与氮杂芳族部分的性质和外部阴离子的对映体纯度都有显著的依赖性。这些 Ca(2+)配合物可用作测定手性阴离子对映体过量的有效探针。环戊二烯-Ca(2+)配合物还通过离子对相互作用与非配位的 Δ-TRISPHAT 阴离子相互作用，实现从阴离子到手性金属配合物的手性转移；由于与 Δ-TRISPHAT 阴离子的离子对相互作用，配合物 1-Ca(2+)和 3-Ca(2+)都在香豆素发色团区域明显显示出诱导的 CD 信号。因此，八齿环戊二烯配体和金属中心的适当组合证明了有效的手性转移。
